Wireless communication comprises that a transmitter, using an antenna, transmits information encoded in electromagnetic waves to a receiver, which detects the electromagnetic waves and is enabled to decode the information encoded in said waves. A radio path disposed between the transmitter and the receiver may affect the wireless communication. For example, where the receiver is in a line of sight of the transmitter, the electromagnetic waves may propagate directly from the transmitting antenna to the receiving antenna. In this case, the radio path may be a straight line from the transmitter to the receiver.
In cases where the receiver is not in a line of sight from the transmitter, the radio path may become more complex than a straight line. For example, the radio path may comprise reflections from buildings or natural features, such that the receiver may detect at least one reflected ray originating from the transmitter. A received signal strength may depend on whether strong reflected rays from the transmitter are incident on the receiving antenna. To increase the likelihood that strong rays are incident, the receiver may be furnished with more than one antenna, so that as long as at least one antenna detects a strong ray the transmitted information may be successfully decoded from the electromagnetic waves. Signals from each receiving antenna may be combined to harvest any detected rays.
In addition to multiple receiving antennas, also the transmitter may be furnished with more than one antenna. The effect of this is that the number of transmitted, and thus also reflected rays is increased as there are more than one transmitting antenna, thus also the likelihood that at least one of the rays is detected at the receiver is increased.
Multiple-input multiple-output, MIMO, transmission in general refers to multiple antennas at both the transmitter and the receiver. MIMO may enable, depending on implementation, at least one of beamforming, spatial multiplexing and diversity coding. Beamforming may comprise, for example, that a same signal is transmitter from more than one antenna with phase and amplitude weighting to improve reception at a receiver. Spatial multiplexing may comprise, for example, that a datastream is split into more than one lower-datarate signal, each lower-datarate signal being transmitted from a separate antenna. Diversity coding may comprise, for example, that the same datastream is transmitted from more than one antenna such that transmissions from different antennas are coded orthogonally to each other.